The present invention relates generally to group II-VI semiconductors, for example for use in semiconductor devices such as light emitting diodes (LEDs), laser diodes (LDs), detectors, and transistors and more particularly to p-type group II-VI semiconductors, preferably satisfying a formula Zn1-a-b-cMgaCdbBecO1-p-qSpSeq, wherein a=0˜1, b=0˜1, c=0˜1, p=0˜1, and q=0˜1 and with a group IV element dopant, for example for use in said semiconductor devices.
Among said group II-VI semiconductors, the oxide semiconductors satisfying the formula Zn1-a-b-cMgaCdbBecO, wherein a=0˜1, b=0˜1, and c=0˜1, especially ZnO, have recently received significant attention for use in semiconductor devices including but not limited to LEDs. LDs, photodetectors, and transistors. As a wide bandgap semiconductor having an energy gap of 3.23 eV, ZnO can cover the spectral range from visible to deep UV through use of ZnCdO, ZnMgO, and ZnBeO alloys. A notable advantage of said ZnO is its high exciton binding energy, 60 meV, which is 2.3 times of the thermal energy at room temperature (RT). As a result, light emission and detection from ZnO and most of said alloys are excitonic at and above RT. This enables higher internal quantum efficiency and thermal stability than currently available LEDs, LDs, and diode photodetectors covering the same spectral range from visible to deep UV. In addition, said ZnO is transparent in the visible spectral region, has a higher mobility than amorphous silicon, and can be deposited at temperatures lower than those for polycrystalline silicon. These make said ZnO an ideal semiconductor for fabricating transparent, thin film transistors. These transistors have many applications in transparent electronics such as flat panel display and other transparent, flexible electronic circuits.
Most members of said group II-VI semiconductors, preferably satisfying the formula Zn1-a-b-cMgaCdbBecO1-p-qSpSeq, wherein a=0˜1, b=0˜1, c=0˜1. p=0˜1, and q=0˜1, have a proximity or a relevance in crystal structure. The aforementioned advantages and applications related to ZnO may thus be extended to many of these members.
Since said semiconductor devices are mostly pn junction devices, n- and p-doping techniques are required to form the corresponding n- and p-type semiconductor components in said devices. Formation of n-type ZnO components is not considered to be an issue, because ZnO is known to be intrinsically n-type and several group III or VII elements in the Periodic Table of Elements, including Al, Ga, In, and F, have already been identified as effective shallow donors in ZnO. Yet, formation of reliable p-type ZnO components for use in said devices is still immature, though a few examples of doping utilizing group V elements in the ZnO films have been reported to be promising for formation of p-type films.
U.S. Pat. No. 6,624,441B2 (“Homoepitaxial Layers of p-Type Zinc Oxide and the Fabrication Thereof”, H. E. Cantwell and D. B. Eason, U.S. Pat. No. 6,624,441B2, incorporated by reference herein), originally assigned to Eagle-Picher Technologies, LLP and then transferred to the assignee of this application, ZN Technology, Inc., describes a method to achieve p-type conduction in ZnO films by doping nitrogen. An embodiment of the invention uses molecular beam epitaxy (MBE) technique for the p-type ZnO film growth and employs a RF plasma source for generation of atomic nitrogen flux for the p-doping. The nitrogen atoms, after incorporation into the ZnO host lattice, form shallow acceptors in said ZnO films, resulting in p-type conduction. Using this method, p-type ZnO films having hole concentrations of ˜9×1016 cm−3 and mobilities of about 2 cm2/V·s have been successfully grown. A well known drawback with said method to achieve p-type ZnO films is relatively low solubility of nitrogen in crystalline ZnO films or bulk crystals at elevated temperatures (“Repeated Temperature Modulation Epitaxy for p-Type Doping and Light-Emitting Diode Based on ZnO”, A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S. F. Chichibu, S. Puke, Y. Segawa, H. Ohno, H. Koinuma, and M. Kawasaki, Nature Materials, Vol. 4, January 2005, page 42, incorporated by reference herein). As a result, p-type ZnO films or bulk crystals may only be grown at low growth temperatures using nitrogen as the p-type dopant.
Another example is shown by U.S. Pat. No. 6,342,313B1, which discloses a method to grow a p-type ZnO film on a p-type GaAs substrate using pulsed laser deposition (PLD) process (“Oxide Films and Process for Preparing Same”, H. W. White, S. Zhu, and Y. Ryu, U.S. Pat. No. 6,342,313B1, incorporated by reference herein). According to the patent, the group V element As from the p-type GaAs substrate diffuses into the ZnO film during the PLD process, resulting in p-type conduction of the grown ZnO film. This method employs arsenic as the p-type dopant element. The oxide of arsenic is known to be extremely toxic to humans and therefore this method may not be environmentally friendly. In addition, said PLD process also has some difficulties in in-situ manipulation of dopant concentration and alloy composition.
In addition, Xiu, et al reported that p-type ZnO films could be grown on n-type silicon substrates by doping with another group V element, antimony, in the ZnO films using MBE (“High-Mobility Sb-Doped p-Type ZnO by Molecular-Beam Epitaxy”, F. X. Xiu, Z. Yang, L. J. Mandalapu, D. T. Zhao, and J. L. Liu, Appl. Phys. Lett. 87, 152101 (2005), incorporated by reference herein). Yet the mechanism leading to the p-type conduction has not been well understood because antimony, as well as arsenic, is generally considered to form deep levels in ZnO. A model recently proposed, based on first-principles calculations by Sukit Limpijumnong et al, shows that a defect complex, two Zn vacancies teaming with an antimony (or arsenic) atom on a Zn site in the ZnO lattice, may act as a shallow acceptor, providing a possible theoretical basis for the aforementioned p-doping approach used by Xiu et al. (“Doping with Large-Size-Mismatched Impurities: The Microscopic Origin of Arsenic and Antimony-Doped p-Type Zinc Oxide”, S. Limpijumnong, S. B. Zhang, S.-H. Wei, and C. H. Park, Phys. Rev. Lett. 92, 155504 (2004), incorporated by reference herein). With this approach, whether said defect complexes is reliable for long term uses in said semiconductor devices would possibly be an issue and, may need to be further examined.
In spite of some progress recently made in formation of p-type ZnO films, preferably p-type ZnO films are reliable and reproducible, have high net hole concentrations, and can be made in a wide temperature range for uses in said semiconductor devices.
Regarding the use of group V elements, a group V element in general has a significantly higher vapor pressure than a group IV element in the same row of the Periodic Table of the Elements as said group V element. Usually a dopant element having a higher vapor pressure is more difficult to be doped into a host material (said group II-VI semiconductor herein) at high temperatures than a dopant element having a lower vapor pressure. Therefore, said group V element may not be a dopant as effective as said group IV element at high temperatures, while most semiconductor devices require high crystalline p-type semiconductors that have to be formed at high temperatures.